cos(n/7)cos(2n/7)cos(3n/7)

[sinπ/7*cosπ/7*cos2π/7*cos3π/7]/sinπ/7=[sin2π/7*cos2π/7*cos3π/7]/2sinπ/7==[sin4π/7*cos3π/7]/4sinπ/7=[sin(π-3π/7)*cos3π/7)/4sinπ/7==[sin3π/7*cos3π/7]/4sinπ/7=(sin6π/7)/8sinπ/7=[sin(π-π/7)]/8sinπ/7==(sinπ/7)/8sinπ/7=1/8